The true circularity of these objects appears at first view a remarkable feature. But it ceases to be so if we suppose them to have been produced by some very concentrated sublunar force of an upheaving nature, and if only we admit the homogeneity of the moon’s crust. For if the crust be homogeneous, then _any_ upheaving force, deeply seated beneath it, will exert itself _with equal effects at equal distances from the source_: the lines of equal effect will obviously be radii of a sphere with the source of the disturbance for its centre, and they will meet a surface over the source in a circle. This will be evident from Fig. 32, in which a force is supposed to act at F below the surface s s s s. The matter composing s s being homogeneous, the action of F will be equal at equal distances in all directions. The lines of equal force, F_ f_, F _f_, will be of equal length, and they will form, so to speak, radii of a sphere of force. This sphere is cut by the plane at _s s s s_, and as the intersection necessarily takes place everywhere at the extremity of these radii, the figure of intersection is demonstrably a circle (shown in perspective as an ellipse in the figure). Thus we see that an intense but extremely confined explosion, for instance, beneath the moon’s crust must disturb a _circular_ area of its surface, if the intervening material be homogeneous. If this be not homogeneous there would be, where it offered _less_ than the average resistance to the disturbance, an outward distortion of the circle; and an opposite interruption to circularity if it offers _more_ than the average resistance. This assumed homogeneity may possibly be the explanation of the general circularity of the lunar surface features, small and great.
We confess to a difficulty in accounting for such a very local generation of a deep-seated force; and, granting its occurrence, we are unprepared with a satisfactory theory to explain the resultant effect of such a force in producing a raised ring at the limit of the circular disturbance. We may indeed, suppose that a vast circular cake or conical frustra would be temporarily upraised as in Fig. 33, and that upon its subsidence a certain extrusion of subsurface matter would occur around the line or zone of rupture as in Fig. 34. This supposition, however, implies such a peculiarly cohesive condition of the matter of the uplifted cake, that it is doubtful whether it can be considered tenable. We should expect any ordinary form of rocky matter subjected to such an upheaval to be fractured and distorted, especially when the original disturbing force is greater in the centre than at the edge, as, according to the above hypothesis, it would be; and in subsiding, the rocky plateau would thus retain some traces of its disturbance; but in the circular areas upon the moon there is nothing to indicate that they have been subjected to such dislocations.